1. Field of the Invention
This invention relates to a switch having a function of switching load current and another function of limiting excessively high current such as short-circuit current.
2. Description of the Prior Art
Various switches of the type mentioned are conventionally known. An exemplary one of such conventional switches is disclosed, for example, in West German Patent Laid-Open Application No. 3713412 and shown in FIG. 16. Referring to FIG. 16, the switch shown includes an arc box 1 in which a movable contact element 2 is accommodated. A pair of movable contacts 3A and 3B are secured to the opposite end portions of the movable contact element 2 for contacting with a pair of fixed contacts 4A and 4B, respectively. The fixed contacts 4A and 4B are secured to a pair of fixed contacts elements 5A and 5B, respectively, which are, in turn, secured to a pair of bases 6A and 6B by means of screws not shown, respectively.
The arc box 1 has a guide 7 fixedly mounted in the inside thereof and further includes a second operating rod 8 having a first portion 8A fitted for vertical movement in the guide 7 and a second portion 8B integrally formed at and extending laterally from a lower end of the first portion 8A. The movable contact element 2 is located at the other or upper end of the first portion 8A of the second operating rod 8, and the movable contact element 2 and second operating rod 8 are normally urged downwardly by a first spring 9 fitted in the guide 7. A pair of sets or arc-extinguishing plates 11A and 11B made of a magnetic metal material are disposed on the opposite sides of the movable contact element 2 for extinguishing arcs 10A and 10B which may be produced between the movable contacts 3A and 3B and fixed contact elements 4A and 4B, respectively.
An operating mechanism box 12 is secured to the bottom of the arc box 1 and has an electromagnet 13 accommodated therein. The electromagnet 13 includes a fixed iron core 13A, a movable iron core 13B and a coil 13C. A first transmitting member 14 is secured to the movable iron core 13B of the electromagnet 13. A second transmitting member 15 is supported for pivotal motion around a fixed shaft 15A and normally urged at an end portion thereof by a second spring 16 so that the end portion thereof is held in contact with a top end of the first transmitting member 14 while the other end portion thereof is held in contact with a lower face of the second portion 8B of the second operating rod 8.
An electromagnetic driving device 17 is located in the operating mechanism box 12 and includes a fixed iron core 17A, a movable iron core 17B, a coil 17C, a biasing spring 17D and a first operating rod 17E. The opposite ends of the coil 17C are electrically connected to the first fixed contact 5A and a first terminal 18A by way of a pair of first and second lead wires or conductors 19A and 19B, respectively. A second terminal 18B is electrically connected to the second contact 5B.
In operation, when the coil 13C of the electromagnet 13 is energized, the movable iron core 13B of the electromagnet 13 is attracted to the fixed iron core 13A to move the first transmitting member 14 secured thereto upwardly. Thereupon, the second transmitting member 15 is pivoted in the counterclockwise direction in FIG. 16 around the fixed shaft 15A by the first transmitting member 14 to such a position as shown in FIG. 17. Consequently, the movable contact element 2 and second operating rod 8 are moved down by the urging force of the first spring 9 under the guidance of the guide 7. As a result, the movable contacts 3A and 3B on the movable contact element 2 are contacted with the fixed contacts 4A and 4B, respectively, as seen in FIG. 17 to allow load current to flow between the first and second terminals 18A and 18B by way of the movable contact element 2. Such load current also flows through the coil 17C of the electromagnetic driving device 17.
Then, when the coil 13C of the electromagnet 13 is deenergized, the first and second transmitting members 14 and 15, movable iron core 13B of the electromagnet 13, second operating rod 8 and movable contact element 2 are returned to their respective home positions shown in FIG. 16 by the urging force of the second spring 16. Upon upward movement of the movable contact element 2 then, arcs 10A and 10B may be produced between the movable contacts 3A and 3B and fixed contacts 4A and 4B, respectively. The arcs 10A and 10B are attracted to and deformed by the magnetic metal arc-extinguishing plates 11A and 11B as indicated at 10C and 10D, respectively. Such arcs 10C and 10D are cooled by the metal arc-extinguishing plates 11A and 11B, and consequently, they are extinguished at a zero point of ac current.
If a short-circuit accident happens in such condition of the switch as shown in FIG. 17, then high current will flow through the coil 17C of the electromagnetic driving device 17. Consequently, the movable iron core 17B of the electromagnetic driving device 17 is attracted to the fixed iron core 17A to move the first operating rod 17E upwardly to such a position as shown in FIG. 18. Accordingly, the second operating rod 8 and movable contact 2 are moved upwardly by the first operating rod 17E. As a result, arcs 10A and 10B may be produced between the movable contacts 3A and 3B and fixed contacts 4A and 4B, respectively. Also in this instance, the arcs 10A and 10B are attracted to and deformed or elongated by the magnetic metal arc-extinguishing plates 11A and 11B into such arcs as indicated at 10C and 10D in FIG. 16. Consequently, the resistance, of the arcs increase, and as a result, the short-circuit current is limited. Such short-circuit current is interrupted by a breaker not shown.
While conventional switch is constructed and operates in such a manner as described above, since the second operating rod 8 is disposed in a spaced relationship over a significantly great distance from an axial line X-Y of the first operating rod 17E, the second operating rod 8 may be moved up in an inclined condition in the guide 7 as seen in FIG. 18. In this instance, a great frictional force is produced between the second operating rod 8 and guide 7 and will retard the upward movement of the second operating rod 8 thereby to lower the speed of separating movement of the movable contacts 3A and 3B from the respective fixed contacts 4A and 4B. Here, if it is assumed that the short-circuit current then is, for example, 100 kA in effective value, then the rising rate of the short-circuit current is very high at 5.3.times.107 A/sec, and accordingly, the short-circuit current increases suddenly. Accordingly, even if the movement of the second operating rod 8 is retarded only by 1/1,1000 second, the short-circuit current will increase by 53 kA. Therefore, a breaker connected in series to the switch is required to have a high interrupting capacity. Further, since the arc energy in the switch is very high, possible damage to the switch may be very serious. Besides, the amount of conductive hot gas to be discharged from the switch is increased by such high arc energy and may cause a ground-fault accident or may likely hurt a human body. In addition, the internal pressure of the switch may be increased to damage or destroy the switch.